Take the heat and add in miles of up-hill walking and you get: HEAT STROKE!!!! Take it easy on the trails, and wear sunscreen or light, long clothing to protect against the sun. It gets really hot in the canyon no matter how cool on the rims (unless in the winter).
Even on the dry and usually hot South Rim, the weather can change all of a sudden. I recall a trip to Hermit´s Rest - the day had started brilliantly and hot, and then cold air came in. Gosh, was I cold. So don´t forget to take a windjacket along.
4. Watch out for lighting. During the summer months, thunderstorms form all of the time. Bring rain gear to keep yourself dry during a sudden downpour. If there is lightning, stay inside a solid building or stay inside a HARD TOP vehicle (convertibles won't keep your safe), roll up all windows, close all doors, and don't touch anything metal inside. I don't know why they say this but just listen to them anyway... don't hide in caves.